// Copyright (C) 2023 The Qt Company Ltd.
// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GFDL-1.3-no-invariants-only

/*!
    \example 3d/widgetvolumetric
    \meta tags {Graphs, QCustom3DVolume, Custom Item}
    \examplecategory {Data Visualization}
    \title Volumetric Rendering
    \ingroup qtgraphs_examples_3D
    \brief Rendering volumetric objects.

    \e {Volumetric Rendering} shows how to use QCustom3DVolume to display volumetric data.

    \image widgetvolumetric-example.png
           {3D volumetric terrain with colored elevation layers and slice controls}

    \include examples-run.qdocinc

    \section1 Initializing Volume Item

    The QCustom3DVolume items are special custom items (see QCustom3DItem), which can be used
    to display volumetric data. The volume items are only supported with orthographic projection,
    so first make sure the graph is using it:

    \snippet 3d/widgetvolumetric/volumetric.cpp 0

    Create a volumetric item tied to the data ranges of the axes:

    \snippet 3d/widgetvolumetric/volumetric.cpp 1

    Indicate that the scaling of the volume should follow the changes in the data ranges by setting
    the QCustom3DItem::scalingAbsolute property to \c{false}. Next, define the internal contents of
    the volume:

    \snippet 3d/widgetvolumetric/volumetric.cpp 2

    Use eight-bit indexed color for the texture, as it is compact and makes it easy to adjust the
    colors without needing to reset the whole texture. For the texture data, use the data created
    earlier based on height maps.
    Typically, the data for volume items comes pregenerated in the form of a stack of images, so
    the data generation details can be skipped. For more information about the actual data
    generation process, see the example code.

    Since eight-bit indexed colors are used, a color table is needed to map the eight-bit color
    indexes to actual colors. In a typical use case, you would get the color table from the source
    images instead of using one manually defined:

    \snippet 3d/widgetvolumetric/volumetric.cpp 3

    To have an option to show slice frames around the volume, initialize their properties.
    Initially, the frames will be hidden:

    \snippet 3d/widgetvolumetric/volumetric.cpp 4

    Finally, add the volume as a custom item to the graph to display it:

    \snippet 3d/widgetvolumetric/volumetric.cpp 5

    \section1 Slicing into the Volume

    Unless the volume is largely transparent, you can only see the surface of it, which is often
    not very helpful. One way to inspect the internal structure of the volume is to view the slices
    of the volume. QCustom3DVolume provides two ways to display the slices. The first is to show
    the selected slices in place of the volume. For example, to specify a slice perpendicular to
    the X-axis, use the following method:

    \snippet 3d/widgetvolumetric/volumetric.cpp 6

    To display the slice specified above, the QCustom3DVolume::drawSlices property must also be set:

    \snippet 3d/widgetvolumetric/volumetric.cpp 7

    The second way to view slices is to use the QCustom3DVolume::renderSlice() method, which
    produces a QImage from the specified slice. This image can then be displayed on another widget,
    such as a QLabel:

    \snippet 3d/widgetvolumetric/volumetric.cpp 8

    \section1 Adjusting Volume Transparency

    Sometimes, viewing just the slices doesn't give you a good understanding of the volume's
    internal structure. QCustom3DVolume provides two properties that can be used to adjust the
    volume transparency:

    \snippet 3d/widgetvolumetric/volumetric.cpp 9
    \dots
    \snippet 3d/widgetvolumetric/volumetric.cpp 10

    The QCustom3DVolume::alphaMultiplier is a general multiplier that is applied to the alpha value
    of each voxel of the volume. It makes it possible to add uniform transparency to the already
    somewhat transparent portions of the volume to reveal internal opaque details. This multiplier
    doesn't affect fully opaque colors, unless the QCustom3DVolume::preserveOpacity
    property is set to \c{false}.

    An alternative way to adjust the transparency of the volume is to adjust the alpha values of the
    voxels directly. For eight-bit indexed textures, this is done simply by modifying and resetting
    the color table:

    \snippet 3d/widgetvolumetric/volumetric.cpp 11

    \section1 High Definition vs. Low Definition Shader

    By default, the volumetric rendering uses a high-definition shader. It accounts for each
    voxel of the volume with the correct weight when ray-tracing the volume contents, providing an
    accurate representation of even the finer details of the volume.
    However, this is computationally very expensive, so the frame rate suffers.
    If rendering speed is more important than pixel-perfect accuracy of the volume contents, take
    the much faster low-definition shader into use by setting QCustom3DVolume::useHighDefShader
    property \c{false}. The low-definition shader achieves the speed by making compromises on
    accuracy, so it doesn't guarantee that every voxel of the volume will be sampled. This can lead
    to flickering or other rendering artifacts on the finer details of the volume.

    \snippet 3d/widgetvolumetric/volumetric.cpp 12

    \section1 Example Contents
*/
